1. Field of the Invention
The present invention relates to a crystallization system, and more particularly to a crystallization system and method for producing sodium aluminum fluoride cryolite from hydrofluoric acid waste.
2. Description of the Prior Art
High concentration hydrofluoric acid (HF, 49%) or hydrofluoric acid mixture are widely used for etching purpose in science and technology fields, such as semiconductor (IC), liquid display panel (TFT-LCD), and solar cells. Hence, large quantity of waste hydrofluoric acid of various concentrations (from 49% to 0.5%) is produced and needs to be waste treated.
The existing methods for disposal of waste hydrofluoric acid include treating with Calcium compounds, including CaO Ca(OH)2 CaCl2 and etc, which will produce CaF2 after reacting with the fluoride of the waste. Or, the waste hydrofluoric acid can be added with specific chemical agents to solidify and remove the fluoride. The above two methods are costly and difficult to implement. Besides, the volume of the CaF2 sludge is very large, and the percentage of the Caf2 whose purity is high enough to meet recycling standard is very low. The latter method can produce Na3AlF6, but it has to use PAC (Polyalumi-num Chloride) as coagulant, which will affect the crystalline purity of the cryolite.
It is to be noted that sodium aluminum fluoride cryolite can be generated from hydrofluoric acid and sodium aluminate through the following chemical equations: 12HF+3NaAlO2→Na3AlF6+2AlF3+H2O. Based on the above method, the inventor of this application has filed a patent application in Taiwan, which is titled “crystallization system and method for producing sodium aluminum fluoride cryolite from hydrofluoric acid waste”, as shown in FIG. 1. The crystallization system a sodium aluminate chemical tank V2 and a high concentration of hydrofluoric acid waste tank V1, from where the liquid wastes flow through a batch quantity control section F1, F2, through a constant flow and uniform dosing section A which is located at the top portion of the reaction tank of the crystallization system, and a batch crystallization reaction section R which is located at the lower portion of the reaction tank. The constant flow and uniform dosing section A is provided with dispersing aperture of batch dosing of sodium aluminate into the hydrofluoric acid waste. The mixture is crystallized on standing and then moved to the microfiltration section S to remove cryolite, and then the filtered liquid waste flows to a low concentration hydrofluoric acid wastewater treatment section W for further processing.
The purity of the obtained cryolite is as high as 96%, however, the factor affecting the cryolite purity not only involves the way the hydrofluoric acid and sodium aluminate contact and react, but also includes the percentage of impurities contained in the solutions, PH value of the solutions, and etc.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.